An important issue in the storage of a gas, such as natural gas, is how gas which is at low density under normal temperature and pressure can be efficiently stored at high density. Even among natural gas components, butane and similar gases can be liquefied at normal pressure by pressurization at a relatively low pressure (CNG), but methane and similar gases are not easily liquefied by pressure at normal temperature.
One method that has conventionally been used as a method for storage of such gases which are difficult to liquefy by pressure at near normal temperature, is liquefaction while maintaining cryogenic temperature, as in the case of LNG and the like. With this type of gas liquefaction system it is possible to store a 600-fold volume at normal temperature and pressure. However, in the case of LNG for example, a cryogenic temperature of −163° C. or below must be maintained, inevitably leading to higher equipment and operating costs.
An alternative being studied is a method of storing gas by adsorption (ANG: adsorbed natural gas) without special pressure or cryogenic temperature.
In Japanese Examined Patent Publication No. 9-210295 there is proposed an adsorption storage method for gas such as methane and ethane in a porous material such as activated carbon at near normal temperature, in the presence of a host compound such as water, and this publication explains that large-volume gas storage is possible by a synergistic effect of the adsorption power and pseudo-high-pressure effect of the porous material and formation of inclusion compounds with the host compound.
However, even this proposed method is not able to realize storage density comparable to that of storage methods using cryogenic temperature, such as with LNG.
The use of activated carbon has been proposed as a gas occluding material for storage of gases that do not liquefy at relatively low pressures of up to about 10 atmospheres, such as hydrogen and natural gas (see Japanese Unexamined Patent Publication No. 9-86912, for example). Activated carbon can be coconut shell-based, fiber-based, coal-based, etc., but these have had a problem of inferior storage efficiency (storage gas volume per unit volume of storage vessel) compared to conventional gas storage methods such as compressed natural gas (CNG) and liquefied natural gas (LNG). This is because only pores of a limited size effectively function as adsorption sites among the various pore sizes of the activated carbon. For example, methane is adsorbed only in micropores (2 nm or less), while pores of other'sizes (mesopores: approximately 2-50 nm, macropores: 50 nm and greater) contribute little to methane adsorption.